Definition
Weight added to an aircraft, either permanently or temporarily, to bring its center of gravity within approved limits or to maintain a desired weight distribution.
Plain English
Extra weight carried on purpose to keep the airplane balanced within safe limits.
Context Anchor
Seen in loading and balance discussions, including when a nose baggage compartment is used to help keep the airplane within its allowed balance range.
Derivation
From an old Scandinavian word meaning 'bare load' -- weight carried not for its own sake but to steady the vessel. The same idea originally kept ships upright; in aircraft it keeps the loaded airplane within its center of gravity range.
Why Pilots Care
Placing the correct amount of ballast prevents the center of gravity from moving outside allowable limits, which would reduce stability or control authority.
Analogy
Ballast works like adding weight to one side of a seesaw so it balances. The weight is not there because you need to carry it somewhere; it is there because its position helps the whole aircraft balance properly.
Intuition Check
Ballast does not mean any random heavy item thrown into the airplane. In aviation, it means planned weight placed in an approved location to help meet balance limits.
Example Sentence 1
With only the pilot on board and no rear passengers, the loading chart called for ballast in the aft baggage compartment to keep the center of gravity within limits.
Example Sentence 2
After removing the ballast, the pilot re-weighed the airplane to confirm the new center of gravity location.